Bacteria can develop antibiotic resistance up to 100,000 times faster when exposed to Roundup and another widely-used herbicide, a study co-authored by a New Zealand scientist has found.

The findings add to a growing body of evidence that herbicides used on a mass industrial scale, but not intended to be antibiotics, can have profound effects on bacteria, the University of Canterbury's Professor Jack Heinemann said.

This could have potentially negative implications for medicine's ability to treat infectious diseases caused by bacteria, at a time world health authorities are warning that by 2050, 10 million people could be dying annually due to antibiotic resistance.

"The combination of chemicals to which bacteria are exposed in the modern environment should be addressed alongside antibiotic use if we are to preserve antibiotics in the long-term."

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The study, just published in scientific journal PeerJ, comes three years after Heinemann and colleagues reported for the first time that formulations were linked to various responses by potentially pathogenic bacteria.

The researchers previously found that exposure to the herbicide products Roundup, Kamba and 2,4-D or the active ingredients alone most often increased resistance, and sometimes also increased susceptibility of potential human pathogens such as Salmonella enterica and E. coli, depending on the antibiotic.

The latest study showed how herbicide formulations combined with antibiotics increased the rate of antibiotic resistance development, by a change in the genetic makeup of the bacteria by a factor of tens of thousands.

"That is why we call the herbicides gasoline for a fire," Heinemann said.

"This is true regardless of whether the herbicide makes the bacteria more resistant or more susceptible to the antibiotic.

"That will surprise many who think that the more toxic an antibiotic is to bacteria, the longer it will work."

This had broad implications not only for the effects of common industrial chemicals on the ecology of our pathogens, he said, but also on the use of combination drug therapies being investigated to prolong or preserve the effectiveness of antibiotics.

He pointed out that antibiotics, used at mass scale in agriculture and medicine, didn't stay where we used them.

They leached away through waste and drains, before forming into concentrations that selected for bacteria resistant to them.

"Agents such as herbicides increase the number and size of environments at this 'Goldilocks' concentration for promoting resistance."

Incidentally, the study comes the same week as a twist in a landmark US court case involving RoundUp.

In August, a San Francisco jury ordered the weedkiller's makers Monsanto to pay hundreds of millions of dollars to a former school groundskeeper, on the basis its product had contributed to his cancer.

But yesterday, a judge said she was now considering striking down the punitive damages and ordering a new trial, triggering a jump in shares in Monsanto's parent company Bayer.

In agricultural New Zealand, where debate about the use of Roundup's active ingredient glyphosate has increased over recent years, the Environmental Protection Authority considered the product safe to use and the initial US ruling hadn't changed that position.

However, Associate Environment Minister Eugenie Sage said at the time of the August decision that she'd be asking the agency to consider adding Roundup to its hazardous substance reassessment list.